tag:theconversation.com,2011:/au/topics/teenager-31073/articlesTeenager – The Conversation2017-10-19T13:51:20Ztag:theconversation.com,2011:article/851572017-10-19T13:51:20Z2017-10-19T13:51:20ZTeens are sleeping less – but there's a surprisingly easy fix<figure><img src="https://images.theconversation.com/files/189309/original/file-20171009-25792-iceli6.jpg?ixlib=rb-1.1.0&amp;q=45&amp;auto=format&amp;w=496&amp;fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-vector/vector-illustration-bored-woman-lying-alone-721523575?src=gmb2eysLqZDfwHUIH_X_RA-1-8">jesadaphorn</a></span></figcaption></figure><p>Something is stealing teens’ sleep.</p>
<p><a href="http://www.sleep-journal.com/article/S1389-9457(17)30350-7/fulltext">In a newly released analysis of two large national surveys</a>, my co-authors and I found that the number of U.S. teens who reported sleeping less than seven hours a night jumped 22 percent between 2012 and 2015. Sleep experts agree that <a href="https://sleepfoundation.org/ask-the-expert/sleep-and-teens-biology-and-behavior">teens need at least nine hours of sleep a night</a>. But by 2015, 43 percent of teens reported sleeping less than seven hours a night on most nights – meaning almost half of U.S. teens are significantly sleep-deprived.</p>
<p>What could have raised sleep deprivation among teens to such unprecedented levels? Some factors are easy to rule out. For example, we found that the amount of time teens spent working, doing homework and participating in extracurricular activities held steady during those years. </p>
<p>But there was one large change in teens’ lives between 2012 and 2015: More owned smartphones.</p>
<h2>It starts as an alarm clock…</h2>
<p>Today’s teens – <a href="http://www.simonandschuster.com/books/iGen/Jean-M-Twenge/9781501151989">whom I call “iGen”</a> – are the first generation to spend their entire adolescence with smartphones. </p>
<p>In our analyses, we found that teens who spent more time online and on social media were more likely to sleep less. Time spent watching television had a much weaker link to fewer hours of sleep, and teens who spent more time with their friends in person or on sports or exercise actually slept more. </p>
<p>Time spent online, however, was the one teen activity that both increased during the 2010s and was linked to shorter sleep, making it the most likely cause of teen sleep deprivation. Seventeen- and 18-year-olds – <a href="http://onlinelibrary.wiley.com/doi/10.1111/cdev.12930/full">who spend more time online than younger teens</a> – were also the most sleep-deprived: The majority, 51 percent, slept less than seven hours on most nights by 2015.</p>
<p>The link between time spent online and less sleep was considerable. Spending five or more hours a day online (vs. one hour) upped the risk of sleeping too little more than 50 percent. Spending three hours a day (vs. one hour) upped the risk nearly 20 percent. </p>
<p>Smartphones – <a href="http://www.pewresearch.org/fact-tank/2017/01/12/evolution-of-technology/">which the majority of Americans owned by the end of 2012</a> – allow mobile and instant internet access. It’s difficult to prove what causes what in an analysis like this, but it seems much more likely that teens’ increased smartphone use between 2012 and 2015 led to less sleep than less sleep leading to more smartphone use. </p>
<p>Why might smartphones cause teens to sleep less? Unlike other electronic devices such as TVs and desktop computers, smartphones (and tablets) are easily carried into the bedroom and held by hand in bed. </p>
<p>Most of the students I interviewed for my book “iGen” told me they kept their phones within reach as they slept, in part, because they all used it as their alarm clock. </p>
<p>Many also told me that their smartphones were the last thing they looked at before they went to sleep at night. That’s a problem, because answering texts and scrolling through social media is mentally and emotionally stimulating, which <a href="http://www.sciencedirect.com/science/article/pii/S0091743516000025">leads to disturbed sleep</a>. Others told me that they also regularly reached for their phones, often just out of habit, when they woke up in the middle of the night. </p>
<p>There’s a physiological response as well: The blue light emitted by smartphones and tablets simulates daylight, <a href="http://www.pnas.org/content/112/4/1232">inhibiting the brain’s production of melatonin</a>, the hormone that helps us fall asleep and stay asleep.</p>
<p>And that’s if teens try to go to sleep at all. </p>
<p><a href="https://theconversation.com/awake-online-and-sleep-deprived-the-rise-of-the-teenage-vamper-34853">A 2014 study</a> found that 80 percent of teens admitted to using their phones when they were supposed to be sleeping – a practice some call “vamping.” Some said they stayed up most of the night when their parents thought they were asleep. </p>
<h2>Some simple limits</h2>
<p>Sleep deprivation can have serious consequences for teens. </p>
<p>Those who don’t sleep enough <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3707878/">perform more poorly in school</a> and are <a href="http://pediatrics.aappublications.org/content/134/3/e921">at greater risk of developing obesity</a>. Sleep deprivation is also linked to mental health issues <a href="http://www.sciencedirect.com/science/article/pii/S0165178108004113">including depression and anxiety</a> among both teens and adults. </p>
<p>When conducting research for my book, I found that iGen teens are more likely to be <a href="http://pediatrics.aappublications.org/content/early/2016/11/10/peds.2016-1878">depressed</a> and anxious than previous generations. If smartphones cause teens to sleep less, and less sleep leads to depression, sleep deprivation might explain why teen depression increased sharply after 2012 – exactly when smartphones became common, and exactly when sleep deprivation began to increase among teens.</p>
<p>What can be done? <a href="http://pediatrics.aappublications.org/content/134/3/642">Later start times at high schools have significant positive impacts</a> on teen sleep, but school start times aren’t something parents and teens can control. </p>
<p>In contrast, limiting smartphone use before bed is a strategy that can be immediately implemented (ideally for the whole family, adults included). A “no phones in the bedroom after bedtime” <a href="https://theconversation.com/wired-and-tired-why-parents-should-take-technology-out-of-their-kids-bedroom-50406">rule</a> can work. If your family uses phones as alarm clocks, buy inexpensive alarm clocks. Put an app on phones that shuts them down during certain hours, or leave phones and tablets in another room overnight. Suggest reading a book, taking a bath or writing in a journal in the hour before bed.</p>
<p>Your teens will probably get some more sleep – and they might find themselves healthier and happier as well.</p><img src="https://counter.theconversation.com/content/85157/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Jean Twenge has received funding from the Russell Sage Foundation and the National Institutes of Health.</span></em></p>The amount of time teens have spent working and participating in extracurricular activities has held steady in recent years. There has, however, been one big change in their lives: smartphones.Jean Twenge, Professor of Psychology, San Diego State UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/789952017-09-05T20:09:09Z2017-09-05T20:09:09ZWhy we remember our youth as one big hedonistic party<figure><img src="https://images.theconversation.com/files/176695/original/file-20170704-12293-18k5znb.jpg?ixlib=rb-1.1.0&amp;q=45&amp;auto=format&amp;w=496&amp;fit=clip" /><figcaption><span class="caption">Vinyl records and cassette tapes, the parties that went with them, and other hedonistic pleasures from our youth can form a big part of our identity years later.</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/download/confirm/196286351?src=MS9fjuLsxmtThe6kVJk0pg-1-14&amp;size=medium_jpg">from www.shutterstock.com</a></span></figcaption></figure><p><em>This is the last article in our three-part series on <a href="https://theconversation.com/au/topics/hedonism-and-health-41470">hedonism and health</a>.</em></p>
<hr>
<p>Are the hedonistic adventures of your late teens and 20s fresh in your memory? Can you easily recall footloose years when school ended and before serious adult life began? Perhaps you enjoyed a few years of partying until dawn, nights of cheap wine, good friends and song. Or maybe it was all so wild that you remember nothing at all.</p>
<p>Bruce Springsteen sang about looking back on these “glory days”; the days before careers, children or other responsibilities <a href="https://reverb.com/au/news/the-science-behind-if-its-too-loud-youre-too-old">took over</a>.</p>
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<figcaption><span class="caption">Bruce Springsteen’s Glory Days captures the days before careers, children or other responsibilities took over.</span></figcaption>
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<p>Of course, not everybody has time or the opportunity to party, or remembers their youth with perfect pleasure. But why do many of us still recall so vividly and tell stories of our hedonistic younger days? Why do such memories remain rosy and important touchstones?</p>
<h2>Memory is selective</h2>
<p>The first reason is that memory is <a href="http://www.sciencedirect.com/science/article/pii/S0749596X05000987">selective</a>. To remember an experience or event we need to pay attention to it. Then we need to rehearse it by thinking or talking about it. Events that are “encoded” in this way are “stored” in our long-term memory. </p>
<p>But not everything we do, say or feel everyday is encoded and stored in memory. We are more likely to encode events that stand out, are highly emotional, mark first-time experiences or represent big changes in our lives: your first ever muddy music festival or a party that got wonderfully out of control.</p>
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<figcaption><span class="caption">“Hold onto 16 as long as you can” - John Mellencamp, Jack and Diane.</span></figcaption>
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<p>Recalling or “retrieving” events from our long-term memory also is motivated. By motivated we mean that remembering some events but not others serves a psychological purpose. We tend to remember events from the past that are consistent with how we want to see ourselves now. Our sense of identity and memories are completely intertwined. </p>
<p>Former party animals thinking about their past selectively remember party animal memories. Each time they think of these memories, instead of memories inconsistent with this picture, they reinforce a particular view of themselves and their hedonistic past. </p>
<p>Memory researchers call this “<a href="http://psycnet.apa.org/psycinfo/2012-30152-001/">retrieval induced forgetting</a>”: by repeatedly rehearsing or practising some memories (“that time I partied all night”), we forget about other related memories (“that time I studied all night”), shaping and reshaping our sense of the past and ourselves.</p>
<h2>Memories of our teenage years matter</h2>
<p>The second reason is a phenomenon known as “<a href="https://www.ncbi.nlm.nih.gov/labs/articles/12507360/">the reminiscence bump</a>”. When we look back over the past, we don’t remember an equal number of events across our lives. Instead, we remember more from our teenage and early adult years. </p>
<p>Memories in this <a href="https://theconversation.com/the-things-you-remember-best-happened-when-you-were-between-15-and-25-heres-why-68792">reminiscence bump</a> overwhelmingly are of positive, not negative, <a href="https://www.ncbi.nlm.nih.gov/labs/articles/12507360/">experiences</a>. Researchers have long speculated why, but one explanation is these are the years when we form a stable <a href="https://link.springer.com/article/10.3758%2FMC.36.8.1403?LI=true">lifelong identity</a>. </p>
<p>Because the events that happen to us in this “bump” are formative and central to how we view ourselves, we tend to remember them well. And because – for most of us – we selectively remember the past to form a positive, optimistic identity, we encode and store positive rather than negative memories.</p>
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<figcaption><span class="caption">“Those were the best days of my life” - Bryan Adams, Summer of ‘69.</span></figcaption>
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<p>Interestingly, the reminiscence bump applies not just to our personal experiences, but also to the <a href="https://link.springer.com/article/10.3758/s13421-016-0647-2">music we recognise and love</a>. This personally significant music usually dates from our teenage and early adult years and can <a href="https://theconversation.com/the-power-of-our-song-the-musical-glue-that-binds-friends-and-lovers-across-the-ages-73593">trigger vivid memories</a> decades later. </p>
<p>So the identity we form in our early adulthood – the wild child – shapes our recollections and helps shape us for the rest of our lives – the former wild child settling down.</p>
<h2>Memory can be a social glue</h2>
<p>The third reason is that memory is inherently <a href="https://www.ncbi.nlm.nih.gov/pubmed/23808866">social</a>. We use memory to build our individual identity, but just as importantly, we use memory to build social bonds, entertain others and teach the next generation (“do what I say, not what I did”).</p>
<p>In fact, researchers have shown that adolescents who can re-tell their parents’ teenage memories and connect them to their own developing sense of identity report higher levels of <a href="https://theconversation.com/remember-when-we-why-sharing-memories-is-soul-food-35542">psychological well-being</a>.</p>
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<figcaption><span class="caption">“A long, long time ago, I can still remember …” - Don McLean, American Pie.</span></figcaption>
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<p>Over time, the exact details of what happened when we were young may become less important than the sense of belonging and shared identity we gain from joint reminiscing and storytelling. </p>
<p>Events become exaggerated, parties become wilder and bands become more amazing as we tell and re-tell stories for <a href="http://journals.sagepub.com/doi/full/10.1111/j.1467-8721.2007.00467.x">different audiences and different purposes</a>: from nostalgic reminiscing at our high school reunion, to introducing our children to Pink Floyd, or starting those tricky parenting conversations about sex, drugs and rock ‘n’ roll.</p>
<p>So we remember and tell stories of our hedonistic past because these events were memorable, unless it was the 60s and you remember nothing! Remembering them also helps us to see ourselves then and now in desirable ways, and sharing these memories binds us to others in important ways. </p>
<p>But were we as wild as we remember? Perhaps or perhaps not. But our memories of more carefree times serve us well.</p>
<hr>
<p><em>Read other articles in our hedonism and health series:</em></p>
<p><em><a href="https://theconversation.com/what-is-hedonism-and-how-does-it-affect-your-health-78040">What is hedonism and how does it affect your health?</a></em></p>
<p><em><a href="https://theconversation.com/hedonism-not-only-leads-to-binge-drinking-its-part-of-the-solution-81751">Hedonism not only leads to binge drinking, it’s part of the solution</a></em></p><img src="https://counter.theconversation.com/content/78995/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Amanda Barnier receives funding from the Australian Research Council.</span></em></p><p class="fine-print"><em><span>Celia Harris receives funding from the Australian Research Council.</span></em></p>Memories of our carefree youth help form our identity today. But memories are selective. So, were we really as wild as we think we were?Amanda Barnier, Professor of Cognitive Science and Australian Research Council Future Fellow, Macquarie UniversityCelia Harris, Australian Research Council Discovery Early Career Research Fellow, Macquarie UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/808922017-07-20T01:57:22Z2017-07-20T01:57:22ZThree ways to help your teenage kids develop a healthier relationship with alcohol<figure><img src="https://images.theconversation.com/files/178560/original/file-20170718-19023-1xl6c6a.jpg?ixlib=rb-1.1.0&amp;q=45&amp;auto=format&amp;w=496&amp;fit=clip" /><figcaption><span class="caption">Parents play an important role in when their teenage children start drinking and their drinking patterns as they grow.</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/download/confirm/633035360?size=medium_jpg">from www.shutterstock.com</a></span></figcaption></figure><p>Lauren, a 15 year old school student, goes to a party at a friend’s house where the parents have supplied alcohol. She drinks too much and vomits in the car on the way home. </p>
<p>While this type of story may be common, it’s not inevitable. Our <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5461699/">new research</a> suggests parents have more of a positive influence on their teenagers’ relationship with alcohol than they realise.</p>
<p>What parents say, how they behave and the messages they send to their teenagers can <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4108600/">help delay</a> when their teenage kids start drinking, which is critical if they are to avoid the harmful and life-long effects of alcohol on the developing brain.</p>
<p>Parental influences also set the path to better <a href="https://www.ncbi.nlm.nih.gov/pubmed/15585043">drinking patterns (and reduced rates of alcohol dependence)</a> <a href="https://www.ncbi.nlm.nih.gov/pubmed/11775078">as their teenage kids</a> <a href="https://www.ncbi.nlm.nih.gov/pubmed/16818840">grow up</a>.</p>
<h2>Why are we so concerned about teenage drinking?</h2>
<p>Teenagers’ brains are <a href="http://www.sciencedirect.com/science/article/pii/S0010945212001839">still developing</a> key pathways for memory, learning, judgement and impulse control. So, damage from alcohol misuse at this critical time in development can lead to <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3960066/">cognitive issues</a> like memory problems and learning difficulties.</p>
<p>This is one reason Australian guidelines <a href="https://www.nhmrc.gov.au/_files_nhmrc/publications/attachments/ds10-alcohol.pdf">recommend</a> people under 18 avoid drinking alcohol altogether and delay starting for as long as possible.</p>
<h2>How big a problem is it?</h2>
<p>Out-of-control teenage parties often make the <a href="http://www.brisbanetimes.com.au/queensland/sunshine-coast-teen-parties-spiral-out-of-control-20161007-grx20r.html">news headlines</a>, so it’s easy to think teenage drinking is a growing problem. </p>
<p>But two <a href="http://www.aihw.gov.au/alcohol-and-other-drugs/data-sources/ndshs-2016/key-findings/">national</a> <a href="http://www.nationaldrugstrategy.gov.au/internet/drugstrategy/Publishing.nsf/content/E9E2B337CF94143CCA25804B0005BEAA/%24File/National-report_ASSAD_2014.pdf">surveys</a> have confirmed drinking rates among school students (aged 12-17) and young adults (aged 18-24) are declining.</p>
<p>So, while young people may think drinking is the norm, <a href="http://www.nationaldrugstrategy.gov.au/internet/drugstrategy/Publishing.nsf/content/E9E2B337CF94143CCA25804B0005BEAA/%24File/National-report_ASSAD_2014.pdf">this is not the case</a>, especially for those under the age of 15 where fewer than 15% report having drunk alcohol in the past month and 8% in the past week.</p>
<p>Nevertheless, alcohol contributes to four of the top five leading causes of death in 15-24 year olds, including <a href="http://www.aihw.gov.au/WorkArea/DownloadAsset.aspx?id=60129552757">suicide</a>, <a href="http://www.aihw.gov.au/WorkArea/DownloadAsset.aspx?id=60129552762">traffic accidents</a>, <a href="http://www.aihw.gov.au/WorkArea/DownloadAsset.aspx?id=60129552765">accidental poisoning</a> and <a href="http://www.aihw.gov.au/WorkArea/DownloadAsset.aspx?id=60129552768">assault</a>. Alcohol in this age group can also lead to <a href="http://www.jsad.com/doi/abs/10.15288/jsa.1998.59.71">sexual risk-taking</a> and is often associated with <a href="http://www.nationaldrugstrategy.gov.au/internet/drugstrategy/Publishing.nsf/content/E9E2B337CF94143CCA25804B0005BEAA/%24File/National-report_ASSAD_2014.pdf">trying smoking or taking illicit drugs</a>.</p>
<p>It’s easy to think our efforts as parents to foster healthier drinking habits in our teenage children is futile. But evidence shows exactly how parents can make a difference. Here are three things you can do to help your teenage kids develop a healthier relationship with alcohol.</p>
<h2>1. Limit availability of alcohol</h2>
<p>Many parents <a href="https://www.ncbi.nlm.nih.gov/pubmed/22763230?dopt=Abstract">believe</a> supplying their children with alcohol in the safe environment of their home teaches them to drink responsibly.</p>
<p>In fact, an <a href="http://www.nationaldrugstrategy.gov.au/internet/drugstrategy/Publishing.nsf/content/E9E2B337CF94143CCA25804B0005BEAA/%24File/National-report_ASSAD_2014.pdf">Australian survey found</a> parents were the most common source of alcohol with 38% of 12-17 year olds who had drunk in the past week indicating their parents gave them their last drink.</p>
<p>Unfortunately, parental supply – whether deliberate or if teenagers drink their parent’s supply behind their back – is associated with <a href="https://bmcpublichealth.biomedcentral.com/articles/10.1186/s12889-017-4472-8">heavier teen drinking</a>. And <a href="https://www.ncbi.nlm.nih.gov/pubmed/20105420">a study from the Netherlands</a> found the more adolescents drank at home, the more they were likely to drink outside of the home, which predicted future problem drinking.</p>
<p>Our study <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5461699/">also found</a> if adolescents thought they could buy alcohol easily, they were more likely to drink regularly. And the more spending money 16-17 year olds had, the more likely they were to drink.</p>
<p>The weight of the evidence is now clear. Allowing children to drink underage and supplying them with alcohol, including at parties, even under the supervision of a parent, <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4108600/">is not recommended</a>.</p>
<h2>2. Set boundaries and clear expectations</h2>
<p>Parents have a vital role to play in setting boundaries and clear expectations about drinking alcohol. Discuss alcohol with your kids and the fact <a href="http://www.aihw.gov.au/alcohol-and-other-drugs/data-sources/ndshs-2016/key-findings/">not everyone drinks</a>, even though it might seem like they do.</p>
<p>Talk about upcoming activities, including parties, and discuss expectations about acceptable and unacceptable behaviour. Talk to other parents and let them know your expectations as it’s important to set <a href="https://www.ncbi.nlm.nih.gov/pubmed/25343647">clear social norms and expectations</a>.</p>
<p>Our study <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5461699/">found</a> adolescents who thought their parents would disapprove of them drinking alcohol were much less likely to drink. This was the case across the whole age spectrum of 12-17 years. </p>
<h2>3. Be a good role model</h2>
<p>Alcohol is the <a href="http://www.aic.gov.au/crime_types/drugs_alcohol/drug_types/alcohol.html">most widely</a> used recreational drug in Australia and most adult drinking (80%) is done <a href="http://www.aihw.gov.au/WorkArea/DownloadAsset.aspx?id=60129549848">in the home</a>. So, we also need to think about our own drinking in front of our children, however uncomfortable the prospect.</p>
<p>Parents are <a href="http://journals.sagepub.com/doi/pdf/10.1080/00048674.2010.501759">important role models</a> for kids when it comes to alcohol. Setting a good example by limiting drinking in front of kids where possible, not making alcohol and drinking a key focal point, having alcohol-free events and cutting back on your own binge drinking are all significant.</p>
<p>This is important because kids who live in families where parents drink on a regular basis around their children <a href="http://journals.sagepub.com/doi/pdf/10.1080/00048674.2010.501759">are more likely</a> to drink more heavily themselves and start at an earlier age.</p>
<h2>Parents are not the only influence, but are still important</h2>
<p>While parents can play a vital role in their children’s relationship with alcohol, they are not the only factor to influence teenage drinking. </p>
<p>Alcohol is <a href="http://fare.org.au/policy/pricing-taxation/">more affordable </a> in Australia than it has been in the past 30 years, and the number of premises selling alcohol has <a href="http://nceta.flinders.edu.au/files/8713/5226/7677/EN463_LLReport_Exec_summary.pdf">increased substantially</a> in the past 15 years. Throw <a href="http://www.theage.com.au/comment/the-age-editorial/excessive-alcohol-advertising-in-sport-is-out-of-bounds-20160522-gp0x0m.html">advertising</a> and <a href="http://www.abc.net.au/news/2017-01-22/doctors-call-for-ban-on-alcohol-sponsorship-of-cricket/8198706">sports sponsorship</a> into the mix and we have some very strong messages that drinking alcohol is the norm. </p>
<p>Yet, the evidence shows parents can make significant and substantial difference in their teenage children’s relationship with alcohol, particularly in not giving them alcohol before they turn 18 and helping them set good behavioural patterns around alcohol now and for later life.</p><img src="https://counter.theconversation.com/content/80892/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Jacqueline Bowden receives funding from SA Government. </span></em></p><p class="fine-print"><em><span>The research centre Robin Room heads receives funding from federal government research bodies, the Foundation for Alcohol Research and Education, VicHealth, Australian state government commissions, the U.S. National Institutes of Health and the World Health Organization. </span></em></p>Teenagers pick up cues about drinking from you and your family. Here's how you can help them develop a healthier relationship with alcohol.Jacqueline Bowden, PhD candidate, School of Psychology, University of Adelaide and Manager, Population Health Research, South Australian Health & Medical Research InstituteRobin Room, Professor and Director, Centre for Alcohol Policy Research, La Trobe UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/650532016-09-08T12:22:47Z2016-09-08T12:22:47ZTeen obesity caused by going into 'power-saving mode'<figure><img src="https://images.theconversation.com/files/137055/original/image-20160908-25260-qcrxnx.jpg?ixlib=rb-1.1.0&amp;q=45&amp;auto=format&amp;w=496&amp;fit=clip" /><figcaption><span class="caption">Evolution&#39;s negative impact.</span> <span class="attribution"><a class="source" href="http://www.shutterstock.com/pic-46843153/stock-photo-board-overweight-teenage-girl-in-front-of-the-tv.html?src=ihdk0gixm547fRChslk6ag-1-81">LittleMiss/Shutterstock.com</a></span></figcaption></figure><p>It is possible that modern teenagers are trapped by a trait which evolved thousands of years ago to help them through puberty, but which now leaves them vulnerable to obesity. </p>
<p>Adolescents need an extra 20-30% energy every day to fuel the growth and changes in body composition that characterise the six years or so of pubertal development. Energy comes from calories in the food they eat, but how could hunter-gatherers guarantee the extra calories they needed as adolescents when their food supply was limited?</p>
<p>We believe they may have unearthed a strategy that worked well for our ancestors, but which does quite the opposite now.</p>
<p><a href="http://www.nature.com/ijo/journal/vaop/naam/abs/ijo2016158a.html">In our research</a>, we have been monitoring a group of children as they progress through childhood from five to 16 years of age (the EarlyBird study). We found, as expected, that more energy was burnt as children got bigger. However, after the age of ten, the calories they burnt unexpectedly fell, despite the fact that they were growing faster than ever. The amount of calories burnt by age 15 was around 25% lower in both boys and girls. Only at 16 years of age, when the growth spurt was over, did the energy spend begin to increase again. </p>
<p>The study has three important qualities: it is longitudinal (which means that it measures the same group of children throughout), its age spread is very narrow (which means that age-related changes can be more accurately identified), and few people dropped out of the study (important statistically).</p>
<p>In a <a href="http://www.ncbi.nlm.nih.gov/pubmed/25913505">publication last year</a>, we described two distinct waves of weight gain; one occurring sometime between birth and five years of age, and the other in adolescence. The early wave affected only some children – the offspring of obese parents – while the later wave in adolescence involved children generally.</p>
<p>Poor parental eating habits passed on to their children seemed a likely explanation for early obesity, but we had no good explanation for the later wave of obesity, until now.</p>
<h2>Mystery solved?</h2>
<p>Energy balance can be thought of as a bank account. Calories are deposited, and calories are spent. Body size (the balance in the account) depends on the difference between the two. So, although the explanation we offer is entirely speculative, and we will never really know because we don’t have the data on our ancestors, the researchers proposed that a downward shift of energy expenditure into “power-saving mode” might help to conserve the calories needed for the growth spurt in puberty.</p>
<p>The energy burnt over 24 hours has two components: voluntary and involuntary. The voluntary component is physical activity, which is easy to understand. What people understand less readily is that the involuntary component is by far the bigger one. Involuntary energy expenditure (so-called resting energy expenditure) is used just to keep alive; to keep the blood temperature at 36.8°C, fuel the brain to think and enable the organs to function. </p>
<p>Involuntary energy expenditure accounts for around 75% of the total calories burnt in a day, which explains why physical activity has a limited impact on obesity. A fall of 25% in resting energy expenditure makes a big hole in the calories burnt each day.</p>
<p>Why does all this matter, and why does it occur? It matters because it makes obesity more difficult to avoid if teenagers are trapped by a long period of low-calorie burn. We don’t know for sure why it occurs, but could speculate that it may be a throw-back to earlier evolutionary times, when calories were scarce but adolescents still needed 25-30% more calories a day to fuel growth and bodily changes. </p>
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<span class="caption">Not as easy as buying a burger.</span>
<span class="attribution"><a class="source" href="http://www.shutterstock.com/pic-337613222/stock-photo-digital-illustration-of-a-group-of-neandertals-hunting-a-bison.html?src=3tDlV1vL0IMw2RjFf_9Avg-1-5">Nicolas Primola/Shutterstock.com</a></span>
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<p>How did hunter-gatherers assure the supply of extra calories needed to reach maturity? It is possible that their bodies adapted by switching down its calorie expenditure, so as to divert the calories to the energy needed to grow. Obesity is a recent problem, and the adaptation now works adversely in a world where calories are cheap and readily available in a highly palatable mix of sugary drinks and calorie-dense foods.</p>
<p>The worst outcome is that adolescents and their families take these findings to mean that they can do nothing about teenage obesity. The best is that a new explanation for teenage obesity leads to better understanding, and an avoidance of the foods that are the cause.</p><img src="https://counter.theconversation.com/content/65053/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Terry Wilkin receives funding from Juvenile Diabetes Research Foundation</span></em></p>Just don't say you can't do anything about it.Terry Wilkin, Professor, University of ExeterLicensed as Creative Commons – attribution, no derivatives.